Comet Hale-Bopp (September 25, 1997)

This is a summary of recent developments around this comet; the previous was published on the ESO Web on August 1, 1997. It is based on information received directly by email and also from IAU Circulars and on other Hale-Bopp WWW pages. A complete list of these Updates is also available.

Richard M. West (ESO) Munich, September 25, 1997

1. Current state

Comet Hale-Bopp is now far South; at this moment, it is located in the constellation of Puppis (The Stern), at -35 o declination and in the middle of the Milky Way band, almost right at galactic latitude 0 o. This constellation is part of the former, very large and unwieldy constellation Argo (The Ship - also known asArgo Navis ); earlier this century, it was split into three separate constellations, Puppis , Carina (The Keel) and Vela (The Sail).

At this time, the comet is 3.09 AU (462 million km) from the Earth and 2.82 AU (422 million km) from the Sun, moving outwards at ever diminishing velocity; it is now `only' 20.5 km/sec. The angular distance in the sky from the Sun is increasing and is now 66 o , but due to its southern position in the sky, it can only be observed well by observers south of the terrestrial equator. However, by good weather conditions, observers up to about 30 o north latitude would still be able to catch a glimpse of the comet close to the horizon; cf. the finder chart by Glenn Ray for observers at this latitude.

If you live in the south and want to follow Hale-Bopp as it slowly fades, you will find very useful information in the account by Alan M. MacRobert (Comet Hale-Bopp Flees Far South).

The most recently reported visual observations, most from Brazil and Australia, indicate a magnitude of about 4.6 - 4.8, not too different from that predicted (if anything, perhaps a little on the bright side). The dust tail is now about 2 degrees long when seen under good viewing conditions (no moon). It will probably be possible to follow the comet with the unaided eye for another 1, perhaps even 2 months.

2. Aftermath of Comet Hale-Bopp

Among the recently published accounts, I would like to mention two excellent contributions, both displayed at the Sky & Telescope website.

The first of these, Hale-Bopp: The Great Comet of 1997 by Edwin L. Aguirre interestingly mentions that, according to a survey of 1,000 adults, by April 9th, 69 percent of Americans had seen the comet for themselves and another 12 percent said they planned to make a point of doing so. That would increase Hale-Bopp's viewership to 81 percent of adults, an incredibly high fraction of the US population. From the experience in Germany, it seems clear that well over half of the population in that country saw Comet Hale-Bopp, although no firm statistics are available there. It may thus well be that with the exception of the Sun, the Moon (and possibly the five classical planets ?), Comet Hale-Bopp is the most observed astronomical object of all times!

An interesting comparison between the brightest comets during the past decades is available in the article on Dueling Comets: Hyakutake vs. Hale-Bopp by Stephen James O'Meara . While he concludes that others may have been brighter or may have appeared larger in the sky, he has little doubt about which one had the greatest impact on the general public!

3. New elements

New orbital elements have recently been published by the Minor Planet Bureau.

You may access these via the Astronomical Headlines' website (click on `Ephemerides and orbital elements for (potentially) observable comets' and then on `Comet 1995 O1 (Hale-Bopp)'). The new elements are based on 2129 observations in the time interval 1993 Apr. 27 - 1997 Aug. 10, and they show the (very small) mean residual of 0.8 arcseconds.

4. Space-based observations

The SWUIS UV/Vis imager experiment has successfully obtained numerous image sequences of Hale-Bopp. A first data reduction has been carried out by the SWUIS team, and some of the first images are available at their website at URL: http://www.boulder.swri.edu/swuis/mission.html. In addition, Alan Stern has flown with an infra-red, wide-field version of SWUIS at 51,000 ft in NASA's WB-57 and obtained 45 minutes of data to characterize SWUIS IR performance at altitude.

Recently, Hal Weaver (Johns Hopkins University, Baltimore) sent out a message to research colleagues via the `Maryland Exploder' which, among others, contained this important information:

A very PRELIMINARY estimate of the water production rate is 3 10 29 molecules/sec at a heliocentric distance of 2.48 AU, which is comparable to what we observed pre-perihelion last fall. (On 23 Sep 1996 at r = 2.97 AU we estimated Q(H 2 O) = 2.1 -2.6 10 29 , while on 18 Oct 1996 at r = 2.69 AU we obtained 2.710 29 ).

The next HST observations are planned for sometime during 11 - 13 November 1997. Information about those observations will also be posted to the above web site.

I strongly recommend all Hale-Bopp fans to visit this very informative site! We all look forward to read about the next observations with the HST.

5. Meetings

The Special Hale-Bopp session at the Kyoto General Assembly of the International Astronomical Union was held on August 23, 1997 went extremely well and had a very large attendance. An overview of the talks and posters is still available on the Web.

The registration to the First International Hale-Bopp Conference at Tenerife in February 1998 is now picking up speed and all potential participants are requested to make themselves known to the organisers soonest possible. Please do not miss the deadlines for registration, etc.

6. Information on IAU Circulars

Since the last Update, the only new information published on the IAU Circulars was on IAUC 6741 (September 12, 1997). Here, D. H. Wooden and collaborators reported 10-micron spectrophotometry with the Wyoming Infrared Observatory 2.34-m telescope (+ NASA/Ames HIFOGS) on Aug. 24: weak silicate feature (9.0-11.6 microns) was detected... Subsequent spectrophotometric measurements on Aug. 29 achieved the same spectral flux densities and shape within the photometric uncertainties. The weak silicate feature detected on Aug. 23 and 28 is 20 times less than that predicted using an [normal] extrapolation....

Readers of these Updates and others interested in the latest astronomical news will be delighted to learn (if you have not already heard about it) that the Central Bureau for Astronomical Telegrams (CBAT) that operates at the Smithsonian Astrophysical Observatory, under the auspices of Commission 6 of the International Astronomical Union (IAU), is now making available the IAU Circulars on the WWW free of charge for personal use by individuals.

The Circulars are posted on the WWW some hours after being issued to the regular subscribers. Thus, if you wish to (continue to) receive automatically these Circulars as they are ready, you must (continue to) subscribe in the usual way. Information about how to subscribe is available.

Why don't you try to have a look at the full text of IAUC 6741, cited above?

7. More new sun-grazing comets found

The scientists who regularly inspect the data from the solar space observatory SOHO keep finding comets very near the Sun. They are referred to assungrazing and most of them apparently fall into the Sun. The latest series of new discoveries is described on IAU Circular 6745 (September 22) which may be accessed on the web as described above. You may inspect some of the images at the website with LASCO movies etc.